1. Field of the Invention
The present invention generally relates to a transistor. More particularly, the present invention relates to a bipolar transistor through a mechanism based on spin polarization.
2. Description of the Related Art
Most semiconductor devices are based on the p-n diode or the transistor. A large class of transistors are the so-called bipolar transistors consisting of back to back p-n diodes either in a p-n-p or n-p-n arrangement. By controlling the chemical potential of the middle region (called the base) the collector current (IC) can be varied, and IC depends on the base voltage (VEB) exponentially.
The development of transistors and its later evolution into the integrated circuit or microchip revolutionized people's daily life and the world. Continuous efforts have been made to find new types of diodes and transistors.
Until recently, the emerging field of magneto electronics has focused on magnetic metals for conducting components (hereinafter “[n]” referring to the nth reference in the attached list of references at the end of the specification). Multilayer magnetoelectronic devices, such as giant magnetoresistive (“GMR”) and magnetic tunnel junction (MTJ) devices, have revolutionized magnetic sensor technology and hold promise for reprogrammable logic and nonvolatile memory applications. The performance of these devices improves as the spin polarization of the constituent material approaches 100%, and thus there are continuing efforts to find 100% spin-polarized conducting materials.
Doped magnetic semiconductors are a promising direction towards such materials, for the bandwidth of the occupied carrier states is narrow. For example, for nondegenerate carriers and a spin splitting of 100 meV the spin polarization will be 98% at room temperature. To date high-temperature (TCurie>100K) ferromagnetic semiconductors such as Ga1-x. Mnx, As are effectively p-doped. Semi-magnetic n-doped semiconductors like BeMnZnSe, however, have already been shown to be almost, 100% polarized (in the case of BeMnZnSe in a 2T external field at 30K). Both resonant tunneling diodes (RTDs) and light-emitting diodes (LEDs) have been demonstrated which incorporate one layer of ferromagnetic semiconductor. It is inevitable that devices incorporating multiple layers of ferromagnetic semiconducting material will be constructed. Note that “ferromagnetic semiconducting material” or “ferromagnetic semiconductor” as used in this specification includes any magnetic and semi-magnetic semiconductors that is a semiconductor and has a spin polarization, which can be affected by or interact with a magnetic field.
Previous research efforts have been directed to the transport properties of specific device geometries based on multilayers of spin-polarized unipolar doped semiconductors, including spin transport in homogeneous semiconductors and calculations of spin filtering effects in superlattices. Reference is made to commonly assigned U.S. patent application Ser. No. 10/014,925 and its entire contents are hereby incorporated by reference thereto into the present patent application.
One motivation for semiconductor spin electronics has been the seminal suggestion of Datta and Das of a “spin transistor”. A burst of recent activity demonstrating controllable fabrication of ferromagnetic semiconductors and their incorporation into heterostructures has led to several additional device suggestions. These include a “spin filter”, “spin-RTD”, “unipolar spin transistor”, “magnetic Zener tunnel diode”, and “magnetic p-n diode”. At the same time, progress on the problem of spin injection into nonmagnetic semiconductors has been reported, both from magnetic semiconductors and from magnetic metals. Hybrid transistor devices incorporating both semiconductors and ferromagnets, such as the spin-valve transistor, have demonstrated magnetoresistances exceeding 300%, but suffer from low efficiency for current transport through the base to the collector. Additional hybrid systems are also under development.